Intertidal surfgrass as an allochthonous resource trap from the subtidal habitat

Abstract

Seagrasses have many important ecological functions in regulating community struc- ture and production in nearshore systems, but little is known about their architectural role as allochthonous resource traps. I experimentally manipulated stands of surfgrass shoots in tide pools of the intertidal rocky bench to determine whether surfgrass trapped 2 types of allochthonous resources (subtidal sea urchins and algal drifts) and thus facilitated 2 recipient consumers (carrion crows and herbivorous gastropods). One experiment revealed that the quantity of sea urchins was significantly higher in tide pools with surfgrass shoots than in those without and was positively correlated with shoot density. In addition, the foraging frequency of crows on sea urchins was significantly higher in tide pools with surfgrass than in those without. Another experiment revealed that the quantity of algal drift was significantly higher in tide pools with surfgrass shoots than in those without, and was positively correlated with mean shoot length. Moreover, the number of gastropods in tide pools with surfgrass was significantly higher than in those without surfgrass, even over the short term. Further- more, gastropod numbers in tide pools with surfgrass were higher 1 yr after the experiment had ended. Therefore, I conclude that surfgrass plays an important architectural role in regulating allochthonous resource input and may influence the behavior and growth of recipient consumers by modifying allochthonous resource subsidies.